Voltage control apparatus for an alternating current generator

ABSTRACT

A voltage control apparatus for an alternating current generator comprising an alternating current generator which is selfexcited, an electric motor for driving the generator, a control rectifier arrangement for exciting the field winding of the generator by rectifying the output voltage of the generator, a phase shifter for controlling the control rectifier, a saturable reactor excited by the output of the generator, a rectifier for rectifying the current flowing through the reactor to supply a control input for the phase shifter, and a resistor connected across the alternating current terminals of the rectifier to bypass the current of the reactor.

United States Patent 1 Kiwaki et al.

[ 1 June 5, 1973 [75] Inventors: Hisakatsu Kiwaki; Hiroshi Sato; TakeoKuwabara, all of Katsuta-shi,

Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Oct. 12,1971 21 Appl. No.: 188,333

[30] Foreign Application Priority Data Oct. 12, 1970 Japan ..45/88829[56] References Cited UNITED STATES PATENTS 3/1970 Dinger ..322/286/1957 Woerdemann ..322/28 X 2/1964 Rosenberry ..322/32 X 6/1966 Hysleret al ..323/89 R Primary Examiner-J. D. Miller Assistant Examiner-H.Huberfeld Attorney-Craig, Antonelli 8!. Hill 571 ABSTRACT A voltagecontrol apparatus for an alternating current generator comprising analternating current generator which is self-excited, an electric motorfor driving the generator, a control rectifier arrangement for excitingthe field winding of the generator by rectifying the output voltage ofthe generator, a phase shifter for controlling the control rectifier, asaturable reactor excited by the output of the generator, a rectifierfor rectifying the current flowing through the reactor to supply acontrol input for the phase shifter, and a resistor connected across thealternating current terminals of the rectifier to bypass the current ofthe reactor.

14 Claims, 7 Drawing Figures PATENTEDJUH 51m 7 SHEEIl 0F 2 FIG.

m T m, E H T v c m L AW 4 E mC 4. 11V N l m GT mmm m w, mm W 1 XU EC IA.C GENERATOR VOLTAGE FIG. 3

/A.C GENERATOR m m Tm mm m E w M U E PW G T A UA U a n I lllm INVENTOR$HISAKATSU KIWAK', HIIDH! SA O TAKI KuwAMM Gala, 0.4mm- Hum ATT'ORNEYSPATENIEBJUN 5197a SHEET 2 OF 2 K4 0 I I EXCITING CURRENTI O PRIM ARYVOLTAGE 8| OF TRANSFORMER OF SATURABLE REAC TOR Etiw ww zm @5232 o 5E8BIAS MMF FIG. 7

INVENTORS HIMKA U Klw KgmRosmsAfo TAK so K UWABARA BY.

v cwa ameoum; H1122 ATTORNEYS BACKGROUND OF THE INVENTION This inventionrelates to a voltage control apparatus for an alternating currentgenerator, particularly for a self-excited alternating currentgenerator.

A conventional alternating current generator driven by an electric motorgenerally has an automatic voltage control apparatus and/or an automaticfrequency control apparatus. These apparatus control the output voltageor the frequency of the output voltage of the alternating currentgenerator to maintain these parameters constant irrespective of thespeed of the motor driving the generator.

Where, however, the output of the generator is supplied to equipmenthaving a considerably large time constant, the control as describedabove is not always necessary. For instance, in the alternating currentgenerator which is used for an auxiliary electric source in electricrailway rolling stock, the motor for driving the generator is suppliedwith voltage derived from a contact wire. The voltage of the contactwire changes within a range from 40 percent to 20 percent for a ratedvoltage. Therefore, the speed of the motor varies in a wide range, sothat the output voltage or the frequency of the voltage of the generatorchanges according to the change in speed of the motor.

On the other hand, cooling and warming equipment supplied by such agenerator have a large time constant. Thus, a small voltage change willscarcely affect the cooling or warming operation. In such a case, it isinadvisable to control the voltage and the frequency to maintain theseparameters constant irrespective of the speed of the motor. The reasonis as follows. The load and the generator show a low impedance for a lowfrequency of the output voltage, and a large current flows through theload and the generator. As a result, it is required that the alternatingcurrent generator has a large current capacity. Further, when thefrequency of the output voltage is high, the impedance becomes high andan extremely high voltage becomes necessary in order to produce therequired current.

The present inventors have investigated a new apparatus, in which theoutput voltage of the generator is controlled in proportion to thefrequency. This apparatus has an automatic voltage control apparatuswhich provides a reference for the output voltage by means of asaturable reactor excited by the alternating current generator. 7

Generally speaking, if the cross-sectional area of a core of thesaturable reactor is A, the number of turns of a winding wound on thecore is N, the saturation magnetic flux density is Bm, the. voltageapplied to the winding is- Ea, and the frequency of the voltage is f,the following equation holds in such the saturable reactor as describedabove:

the reference, it will be proportional to the frequency of the voltage.

This apparatus still has inconvenient features, such as troubles causedby the exciting current of the saturable reactor, a non-linearity of thesaturation characteristic of the core, and an error in the saturationmagnetic flux density or the numberof turns of the windings produced inthe manufacturing process.

SUMMARY OF THE INVENTION the control rectifier, a saturable reactorexcited by the output voltage of the generator, a rectifier forrectifying a current flowing through the reactor in order to supply acontrol input for the phase shifter, and bypassing means provided in analternating current side of the rectifier to bypass the current of thereactor.

Other objects and features of this invention will become apparent uponreading the specification and inspection of the drawings and will beparticularly pointed out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof a preferred embodiment according to this invention;

FIG. 2 is a characteristic of the output current of a saturable reactorvs. the output voltage of an altemating current generator;

FIG. 3 is a wave form diagram of the output current of the saturablereactor;

FIG. 4 is a schematic circuit diagram of a part of another embodimentaccording to this invention;

FIG. 5 is a diagram for explaining the operation of the circuit of FIG.4;

FIG. 6 is a characteristic diagram of a magnetic phase shifter which isused in the circuit of FIG. 1; and

FIG. 7 is a schematic circuit diagram of a part of another embodimentaccording to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, analternating current generator G is driven by an electric motor M. Theelectric source connected to the motor M is omitted from the drawing.The voltage generated by the generator G can be taken from terminals U,V, and W. A control rectifier circuit CR provides a direct current forthe field winding F of the generator G. The rectifier circuit CRcomprises diodes D1, D2, and thyristors T1, T2 An input voltage to therectifier circuit CR is derived from the output ary winding 20 of thetransformer 16 is connected to a main winding 24 of a saturable reactor22. The transformer 16 is provided to match the output-voltage of thegenerator G with the voltage of the control circuit.

The saturable reactor 22 also has an auxiliary winding 26 which iscapable of providing a variable number of turns by means of a changeoverswitch 28. The total number of turns of the saturable reactor 22 isadjusted by changing the changeover switch 28. A resistor 30 restrainsthe exciting current ofthe saturable reactor 22. A resistor 32 bypassesthe exciting current in order to eliminate any undesirable effect causedby a forward voltage drop in the rectifier bridge 34.

A zener diode 36 limits the quantity of a feedback within apredetermined range. Although a more detailed reason is provided later,if the quantity of the feedback exceeds the predetermined range thefeedback controlis brought into a positive feedback range; as a result,the voltage control becomes impossible. A resistor 38 adjusts thequantity of the feedback.

The operation of the apparatus above described is as follows. In thesaturable reactor 22, the equation (1) holds as described already. Eventhough the output voltage of the generator G becomes higher than thevoltage Ea kf), the voltage across the saturable reactor 22 does notexceed the voltage Ea. Further, the characteristic of the output currentof the saturable reactor 22 vs. the voltage of the generator G isideally as shown by the dotted line in FIG. 2. If the resistance of thewindings 24 and 26 is negligibly small, the dotted line becomes morevertical.

The resistance value of the resistor 30 is selected so as to place theoperating point of the saturable reactor 22 at a predetermined point onthe dotted line. The current, which flows through the saturable reactor22 and establishes the operating point thereof, is rectified by therectifier bridge 34 and is always applied to the control input winding12 through the resistor 38. Therefore, a bias current corresponding tothe abovementioned current must be applied to the other control inputwinding 14.

In such a circuit, the output current of the saturable reactor 22changes according to the voltage difference between the output voltageof the generator G and the voltage Ea of the saturable reactor 22. Ifthe output voltage of the generator G is equal to the voltage Ea, thefeedback current applied to the winding 12 is opposed by the biascurrent applied to the winding 14. Then, the phase shifter controls thethyristors T1 and T2 so as to maintain the rated voltage of thegenerator G.

If the output voltage of the generator G exceeds the voltage Ea, thefeedback current increases so that the phase shifter 10 controls thethyristors T1 and T2 so as to decrease the field current, and viceversa.

However, if the resistor 32 is omitted, the output current of thesaturable reactor 22 always flows through the two diodes of therectifier bridge 34. A voltage drop 2ed is produced in these diodes;therefore, the equation (1) changes as follows:

Ea kf-l Zed where the voltage ed is the forward voltage drop per diode(normally about 0.6 l V). The forward voltage drop Zed becomes an errorin the reference voltage,

since the voltage Ea is not proportional only to the frequency f. Theeffect of the voltage 2ed becomes large as the frequency f becomes low.In other words, the circuit without the resistor 32 shows that theoperation is such that the reference voltage becomes always larger thanthe voltage Ea kf) established by the saturable reactor 22 itself by theamount of the error voltage Zed.

Further, a practical characteristic of the output current of thesaturable reactor 22 vs. the voltage of the generator G is as shown bythe solid line in FIG. 2. The characteristic shown by the dotted line isthe ideal, but practically unattainable, characteristic. Namely,although the output voltage of the generator G is below the referencevoltage in this case, the exciting current of the saturable reactor 22,as shown in FIG. 2, flows to the winding 12. An additional currentcorresponding to the exciting current is further required as the biascurrent of the winding 14.

For the purpose of eliminating these inconvenient features, the resistor32 partially bypasses the output current of the saturable reactor 22.The resistance value of the resistor 32 is selected so as to make itsvoltage drop smaller than the forward voltage drop 2ed of the diodes.

The practical characteristic has a non-linear portion in the region ofthe saturation voltage, as shown in FIG. 2, since the characteristic ofa real core is not perfectly angular. The operating point must beselected in the part of the considerably large output current of thesaturable reactor 22. In such a control region, the wave form of theoutput current of the saturable reactor 22 is as shown in FIG. 3. It isapparent from FIG. 3 that the peak value of the output current of thesaturable reactor 22 is remarkably large, as compared with a mean valuethereof. Accordingly, each diode of the rectifier bridge 34 must be ableto endure such a large current; however, this is very uneconomical.However, the resistor 32 also shunts this large current after thesaturable reactor 22 saturates, so that the burden of each diode isreduced thereby. If the resistor 32 is a variable resistor, it is, ofcourse, possible to omit the resistor 38.

There are some other causes which produce an error in the referencevoltage, for example, a dispersion in the saturation magnetic fluxdensity and the number of turns of the windings produced in themanufacturing process.

If the real saturation magnetic flux density of the core of thesaturable reactor 22 is smaller than the expected value Bm byABm, thereference voltage decreases down to (Bm ABm) Ea/Bm. Then, the number ofturns A N of the auxiliary winding 26 is added to the number of turns Nof the main winding 24 by operating the changeover switch 28. The numberof turns A N must be selected as follows:

(Bm A Bm/Bm) X (N- A N/N) =1 On the contrary, in such a case where thesaturation magnetic flux density is larger by A Bm, a part of theauxiliary winding 26 is connected differentially to the main winding 24,so that the number of turns N of the main winding 24 is decreasedfunctionally by the number of turns A N. The polarity of the auxiliarywinding 26, therefore, must be able to be changed in order to add orsubstract the number of turns A N of the auxiliary winding 26 to or fromthe number of turns N of the main winding 24. This is achieved by a wellknown -method. The auxiliary winding 26 can also correct errors in across-sectional area of the core or the number N on the secondary side.Changing the number of turns A N is performed by a changeover switch 21.An adjustment in this case is as follows.

It is assumed here that the relation between a primary voltage e of thetransformer 16, that is the output voltage of the generator G, and thesecondary voltage 2 thereof is as shown in line a in FIG. 5, and thatthe relation between the secondary voltage e and the exciting current Iof the saturable reaction 22 is as shown in line c in FIG. 5. But, thenumber of turns of the main winding is N and the saturation magneticflux density of the core is Bm. In such a case, when e, e the excitingcurrent becomes Io, as shown in the drawing.

Further, if the saturation magnetic flux density becomes Bm A Bm, theexciting current becomes Io since the relation of e vs. I becomes asshown in line d so that the reference voltage changes. Then, if a part AN of the auxiliary winding 19 is connected differentially tothe mainwinding 20, the relation of e, vs. e becomes as shown in line b in FIG.5. As a result, when e e the exciting current may be maintained at thesame value la. The value A N is selected to satisfythe followingequation:

(Bm A Bml Bm) N 'A N2/N2) An adjustment in the transformer 16 asdescribed above can be also performed on the primary side thereof. 7

Since the errors in the saturation magnetic flux density, thecross-sectional area of the core and the winding exist certainly inpractical cases, an extremely large effect is actually produced by anapplication of the correction as mentioned above.

Further, the transformer 16 matches the output voltage of the generatorG with the voltage of the control circuit. If there is no transformer16, a large size core andv a large number of windingturns will berequired in the saturable reactor 22 when the output voltage of thegenerator G is high. These requirements are inconvenient from thestandpoint of weight and economy. The

If the output voltage of the generator G increases for some reason, theoutput current of the saturable reactor 22 increases, so that thecontrol MMF increases and the point P is shifted in a direction wherebythe output of the phase shifter 10 decreases. The-increase of the outputvoltage of the generator G is restrained thereby. In the same manner, ifthe output voltage of the generator G decreases, the point P is shiftedin a direction whereby the output of the phase shifter 10 increases. Theoutput voltage of the generator G increases, therefore. As explainedabove, changes in the output voltage of the generator G are restrainedby the function of the saturable reactor 22 in a normal load condition.

The function of the voltage control, however, is occasionally-lost in aspecial load condition. The aforementioned special load condition meansthat the load has a lagging power factor, such as an induction electricmotor.

As well known, the output voltage (terminal voltage) of an alternatingcurrent generator is decreased by the influence of the armature reactionin a lagging power factor load. In this case, the field current of thegenerator needs to be large in order to maintain a predetermined outputvoltage. In the circuit as shown in FIG. 1, under these conditions, forinstance breaking a full load, the output voltage of the generator Gincreases suddenly. The output of the phase shifter 10 begins todecrease with its own lag. The output voltage of the generator G cannotdecrease suddenly since there is a lag in the field winding F of thegenerator G. Theoperating point of the phase shifter 10, therefore,arrives at a point P beyond the point P The region represented by thepoint P is a positive feedback region. The function of the voltagecontrol is lost in this region, so that the output voltage of thegenerator G increases unusutransformer 16 steps down the output voltageof the generator G in order to eliminate these inconveniences. If theoutput voltage of the generator G is low, the accuracy of control isincreased by stepping up the output voltage of the generator G by meansof the transformer 16.

The magnetic amplifier used as the phase shifter 10 generally has acharacteristic as shown in FIG. 6. The bias magnetomotive force (MMF)applied to the winding 14 and the control MMF'applied to the winding 12are selected so as to position the operating point of the phase shifter10 near a point P as shown in FIG. 6.

ally.

The zener diode 36 is a limiter which is provided to limit an abnormallylarge control MMF, as described above. The zener voltage of this diode36 is selected so as not to be rendered conductive by a normal voltagerise, but becomes conductive in response to an abnormal voltage rise, asdescribed above. By rendering the diode 36 conductive, the phase shifter10 is prevented from making the operating pointenter into the positivefeedback region. Q

At least the two adjacent diodes of the rectifier bridge 34 may bereplaced with zener diodes instead of providing the zener diode 36. Theabnormal voltage rise can be also limited on the alternating currentside of the rectifier bridge 34. FIG. 7 shows a circuit wherein asaturable reactor 40 is connected in parallel to the resistor 32. Thesaturable reactor 40'is not to be saturated by the normal voltage rise,but is saturated by an abnormal voltage rise to restrain the voltagedrop across the resistor 32.

Having thus described the present invention, it is obvious that variousmodifications within the knowledge of workers in the art may be utilizedwithout departing therefrom.

It is to be understood also that although the invention has beendescribed with specific reference to particular 1. A voltage controlapparatus for an alternating current generator having a field windingcomprising a motor in driving connection with said generator, voltagecontrol rectifier means for exciting the field winding of said generatorwith the rectified output voltage of said generator, phase shifter meansfor controlling said control rectifier means in response to an appliedcontrol signal, a saturable reactor excited by the output wherein saidreactor has an auxiliary winding and a main winding, and means forselectively connecting portions of said auxiliary winding in series withsaid main winding so as to change the number of turns and polarity ofsaid reactor.

3. A voltage control apparatus according to claim 1- further comprisinga transformer having a primary winding connected across the output ofsaid generator and a secondary winding connected to said reactor formatching the output voltage of said generator with the voltage forexciting said reactor.

4. A voltage control apparatus accordingto claim 3 wherein saidtransformer has an auxiliary winding and a main winding on the secondaryside, and means for selectively connecting portions of said auxiliarywinding in series with said main winding so as to change the number ofturns.

5. A voltage control apparatus according to claim 1 further comprising alimiter connected to said reactor for limiting the output current ofsaid reactor.

6. A voltage control apparatus according to claim 5 wherein said limiteris a zener diode provided across a direct current output side of saidrectifier.

7. A voltage control apparatus according to claim 5 wherein said limiteris an additional saturable reactor provided across an alternatingcurrent input side of said rectifier.

8. A voltage control apparatus for a self-excited alternating currentgenerator having a voltage control rectifier arrangement connecting theoutput of the generator to its field winding, comprising phase shiftermeans for controlling said control rectifier arrangement in response toan applied control signal to vary the phase angle of conduction thereof,1 control means for generating said control voltage including asaturable reactor excited by the output of said generator and rectifiermeans connected in series with said reactor across the output of saidgenerator and to said phase. shifter means for rectifying the currentflowing through said reactor and supplying the rectified current as saidcontrol signal to said phase shifter means, and

bypassing means connected.. across the alternating current side of saidrectifier means for partially bypassing the current of said reactor.

9. A voltage control apparatus according to claim 8 wherein saidrectifier means is formed by a four diode rectifier bridge and saidbypassing means is formed by a resistor connected across the input ofsaid bridge.

10. A voltage control apparatus according to claim 9 wherein saidresistor has a value to provide a voltage drop smaller than the voltagedrop of the pair of diodes forming the input of said bridge.

11. A voltage control apparatus according to claim 9 wherein anadditional saturable reactor is connected in parallel with saidresistor. v

12. A voltage control apparatus according to claim 9 further comprisinga transformer connected between said generator and said reactor formatching the output voltage of said generator with the voltage forexciting said reactor.

13. A voltage control apparatus according to claim 12 further comprisinga limiter connected to said reactor for limiting the output current ofsaid reactor.

14. A voltage control apparatus according to claim 13 wherein saidlimiter is a zener diode provided across a direct current output sideofsaid rectifier.

1. A voltage control apparatus for an alternating current generatorhaving a field winding comprising a motor in driving connection withsaid generator, voltage control rectifier means for exciting the fieldwinding of said generator with the rectified output voltage of saidgenerator, phase shifter means for controlling said control rectifiermeans in response to an applied control signal, a saturable reactorexcited by the output of said generator, a rectifier connected in serieswith said reactor across the output of said generator and to said phaseshifter means for rectifying the current flowing through said reactorand supplying the rectified current as said control signal to said phaseshifter means, and bypassing means connected to the alternating currentside of said rectifier to partially bypass the current of said reactor.2. A voltage control apparatus according to claim 1 wherein said reactorhas an auxiliary winding and a main winding, and means for selectivelyconnecting portions of said auxiliary winding in series with said mainwinding so as to change the number of turns and polarity of saidreactor.
 3. A voltage control apparatus according to claim 1 furthercomprising a transformer having a primary winding connected across theoutput of said generator and a secondary winding connected to saidreactor for matching the output voltage of said generator with thevoltage for exciting said reactor.
 4. A voltage control apparatusaccording to claim 3 wherein said transformer has an auxiliary windingand a main winding on the secondary side, and means for selectivelyconnecting portions of said auxiliary winding in series with said mainwinding so as to change the number of turns.
 5. A voltage controlapparatus according to claim 1 further comprising a limiter connected tosaid reactor for limiting the output current of said reactor.
 6. Avoltage control apparatus according to claim 5 wherein said limiter is azener diode provided across a direct current output side of saidrectifier.
 7. A voltage control apparatus according to claim 5 whereinsaid limiter is an additional saturable reactor provided across analternating current input side of said rectifier.
 8. A voltage controlapparatus for a self-excited alternating current generator having avoltage control rectifier arrangement connecting the output of thegenerator to its field winding, comprising phase shifter means forcontrolling said control rectifier arrangement in response to an appliedcontrol signal to vary the phase angle of conduction thereof, controlmeans for generating said control voltage including a saturable reactorexcited by the output of said generator and rectifier means connected inseries with said reactor across the output of said generator and to saidphase shifter means for rectifying the current flowing through saidreactor and supplying the rectified current as said control signal tosaid phase shifter means, and bypassing means connected across thealternating current side of said rectifier means for partially bypassingthe current of said reactor.
 9. A voltage control apparatus according toclaim 8 wherein said rectifier means is formed by a four diode rectifierbridge and said bypassing means is formed by a resistor connected acrossthe input of said bridge.
 10. A voltage control apparatus according toclaim 9 wherein said resistor has a value to provide a voltage dropsmaller than the voltage drop of the pair of diodes forming the input ofsaid bridge.
 11. A voltage control apparatus according to claim 9wherein an additional saturable reactor is connected in parallel withsaid resistor.
 12. A voltage control apparatus according to claim 9further comprising a transformer connected between said generator andsaid reactor for matching the output voltage of said generator with thevoltage for exciting said reactor.
 13. A voltage control apparatusaccording to claim 12 further comprising a limiter connected to saidreactor for limiting the output current of said reactor.
 14. A voltagecontrol apparatus according to claim 13 wherein said limiter is a zenerdiode provided across a direct current output side of said rectifier.